1. Field of the Invention
The present invention relates to martensitic stainless steel for a high depth oil well where there exists moist carbon dioxide gas, salinity, and hydrogen sulfide.
2. Description of the Related Arts
Conventionally, high strength carbon steel or low alloy steel has been widely used for oil well pipes. Recently, many attempts have been made to develop high depth oil wells in order to maintain oil resources. Since the high depth oil wells are located under an environment which there exists moist carbon dioxide gas, the conventional carbon steel or low alloy steel has been replaced by high alloy steel, such as 13% Cr martensite steel. The required properties of the high alloy steel are strength, corrosion resistance, and stress corrosion cracking resistance. The steel which satisfies these properties is disclosed in Japanese Examined Patent Publication No. 3391/1986, Patent Application Laid Open Nos. 199850/1983 and 207550/1986. However, as the depth of oil wells is further increased, carbon dioxide, hydrogen sulfide and chloride ion will be present and some oil wells may be exposed to an environment whose temperature exceeds 150.degree. C. The aforesaid steel fails to provide satisfactory corrosion resistance under the environment described above. To comply with this, duplex stainless steel has been used to satisfy the required corrosion resistance.
Since the duplex stainless steel is more expensive compared with 13% Cr steel, therefore, the steel disclosed in Japanese Patent Application Laid Open No. 174859/1986 has been developed to provide more excellent corrosion resistance and economic efficiency compared with the conventional 13% Cr steel.
However, the steel disclosed in Japanese Patent Application Laid Open No. 174859/1985 is high Ni-contained steel and suffers from sulfide stress corrosion cracking resistance. The sulfide stress corrosion cracking resistance is abridged and called SSC hereafter. Since Ni is expensive, there is no marked difference between high Ni-contained steel and the duplex stainless steel in terms of economic efficiency as well. Therefore, it is urgently called for to develop steel whose corrosion resistance is more excellent than 13% Cr steel, and more economically efficient than the duplex stainless steel.